EXAMINATION 1 Chemistry 3A

Transcription

1 1 EXAMINATION 1 Chemistry 3A Name: Key Print first name before second! Use capital letters! SID #: Peter Vollhardt February 18, 2016 GSI (if you are taking Chem 3AL): Please provide the following information if applicable. Making up an I Grade If you are, please indicate the semester during which you took previous Chem 3A and the instructor: Semester Instructor Auditor Please write the answer you wish to be graded in the boxed spaces provided. Do scratch work on the back of the pages. This test should have 14 numbered pages. Check to make sure that you have received a complete exam. A good piece of advice: Read carefully over the questions (at least twice); make sure that you understand exactly what is being asked; avoid sloppy structures or phrases. It is better to be pedantic in accuracy now than sorry later! Good Luck!

2 2 I. [30 Points] Name or draw, as appropriate, the following molecules according to the IUPAC rules. Indicate stereochemistry where necessary (cis, trans). a. 5-Butyl-3,7-diethylundecane b. trans-1,3-bis(1-methylethyl)cyclopentane c. (3,4-Dibromobutyl)cyclohexane d. 5-(1,1-Dimethylethyl)-3-ethyloctane

3 3 e. cis-1,1,4-trichloro-5-fluorocyclooctane

4 4 II. [30 Points] Write the best Lewis structure for each of the following molecules. Remember to assign charges, if any, to atoms! Pauling Electronegativities a. :P N: b. What is the geometry of ClF2 +? Place an x mark into the box next to your answer. Linear Bent x

5 5 c. As the acronym implies, this one is unusual. It has three octet resonance forms. Circle the best one (remember Coulomb s Law!). d. Note: For the following molecule, the basic connectivity is provided. Add multiple bonds, charges, electron pairs, as necessary. There are three octet resonance forms. Circle the best. Enter in the respective boxes the hybridization of the indicated atom, i.e. sp, sp 2, or sp 3. O N C C sp 2 sp 2 sp 2

6 6 III. [30 Points] The ethenyl anion, 2CC, is a strong base that is protonated by + (e.g., from water) to give ethane (ethylene), 2CC2. Ethenyl anion Ethene; pka = 44 a. Draw the orbital on 2CC involved in this bond formation and show its overlap with the appropriate atomic orbital of +. Clearly label these orbitals (e.g. 1s, 2s, 2p, 3s, 3p, sp, sp 2, sp 3, etc.) 1s sp 2 b. Draw the orbital energy splitting diagram for the formation of C24 by the reaction of 2CC with +. Clearly depict the energy levels of the orbitals entering into overlap and label them, and show the resulting bonding and antibonding molecular orbital levels. Place the relevant electrons into the various levels. c. In view of the above, would you consider that hydride addition (e.g. reaction with ) to 2CC would give a stable bond? Explain (one sentence). No, because the antibonding orbital is filled.

7 7 IV. [20 Points] A petroleum researcher wants to explore the possibility of the catalytic transformation of propane + methane to give 2-methylpropane and hydrogen shown below. Propane Catalyst, heat + C º =? K =? 2-Methylpropane D = 104 kcal mol 1 a. Determine the feasibility of this transformation by calculating its º. Enter your answer in to box provided below and show your work. º = ( ) ( ) = kcal mol 1 b. Estimate roughly the equilibrium constant for the equilibrium above at room temperature, using the equation Gº = 1.36 logk and judging that the Sº term is negligible. By roughly is meant a range, for example, 10 1 to Mark the box next to your choice of an answer with an X. 1. K ~ x 2. K ~ K ~

8 8 V. [50 Points] Consider the rotation about the C2 C1 bond in 1-methyl-2- (trimethylsilyl)propane, a molecule we mentioned in class: The Newman projections A F below illustrate the sequential clockwise motion of the backcarbon (C1) in increments of 60. A B C D E F a. Indicate, by circling the appropriate letter, the rotamers that contain substituents that are (with respect to each other) anti or gauche, or rotamers that are eclipsed or staggered: anti: A gauche: A eclipsed: A staggered: A B B B B C C C C D D D D E E E E F F F F

9 9 b. Draw a potential energy diagram for this movement. Start by assigning a relative energy to each rotamer on the diagram below, before drawing the interconnecting curve. A B C D E F A c. Two of the staggered rotamers have the same energy. Show which ones, by placing the appropriate letters in the box. A, C d. Two of the eclipsed rotamers have the same energy. Show which ones by placing the appropriate letters in the box. D, F

10 10 VI. [50 Points] An alternative to Cl2 in radical halogenations of alkanes is chloramine, ClN2, as exemplified below for the chlorination of methane. 60 kcal mol 1 a. Using the bond dissociation energy provided in equation (1) and the Table above, calculate the value for reaction (1). Show your work. (1) : ( ) ( ) = 28 kcal mol 1 b. Is the reaction (1) thermodynamically feasible? Circle your answer. Answer: Yes No

11 11 c. Formulate the two propagation steps for reaction (1). 1 st Propagation step : 2 nd Propagation step : C 3 + Cl N 2 C 3 Cl + N 2 d. The normalized selectivity secondary : primary in chlorinations with ClN2 is 6:1. In the boxes provided, draw the products A and B and their ratio in the monochlorination of 1,1,4,4- tetramethylcyclohexane with ClN2. A (secondary chlorination) B (primary chlorination) Ratio A : B = 4:1

12 VII. [15 Points] a. Using the values in the Table below and the cyclohexane stencils provided, show the structures of the most stable conformers of A, B, and C. For each, calculate the change in free energy on ring flip to the less stable conformer. Make sure to cap off all axial and equatorial bonds with substituents and atoms. Enter your calculated energy of ring flip in the box provided. 12 A: trans-1-bromo-2-fluorocyclohexane F Br +0.8 kcal mol 1 B: trans-1-methyl-3-(1,1-dimethylethyl)cyclohexane kcal mol C 3 1 C: trans-1-iodo-4-methylcyclohexane kcal mol 1

13 13 VIII. [25 Points] 1. Mark the box next to your choice of an answer with an X. Radical bromination of alkanes is more selective than the corresponding chlorinations, because Br is a stronger acid than Cl. the second propagation step is rate-determining. x abstraction occurs via a late transition state. the products are more stable. 2. In the following pair of compounds, circle which one is more acidic. 3. What are the three primary forms of strain present in cycloalkanes? a. angle b. eclipsing or torsional c. transannular

14 14 4. Draw the geometry of ammonia, using the wedged-dashed line notation, and the orbital in which the lone pair resides. Label this orbital as s, p, sp, sp 2, or sp There are four primary factors that control the rates of reactions. Name three. a. energy of activation b. temperature c. concentration (d. probability factor) e did pass Chem 3A The End